This invention relates to an electric vehicle power steering system having an electric servo motor and a control unit for controlling the steering movements of the steerable wheel or wheels of the vehicle by an angle related operation of the servo motor, and a position calibrating device including a disc shaped element rotatable about a central axis in a conjoint relationship with the steerable wheel or wheels as they are turned within an intended steering interval defined by a first extreme position and a second extreme position, wherein the disc shaped element has one or more peripherally extending open and closed portions defining a reference point corresponding to the straight-ahead position of the steerable wheel or wheels as well as points corresponding to the extreme end positions of the steerable wheel or wheels and co-operating with an immobile sensing means for producing digital signals in response to the straight-ahead or extreme end positions passing the sensing means.
In a previously well known type of electric power steering system there is comprised a device having two mechanical switches activated by a rotating member for obtaining indications of the extreme end positions of the steerable wheel or wheels. A problem concerned with this type of device is that the end position switches are activated quite seldom, and there is a risk that they may become malfunctioning due to too little use. This means that the steering interval of the vehicle wheel or wheels may be extended beyond set up safety limits, which would be hazardous to the operation safety of the vehicle, in particular when starting-up the vehicle. In some applications mechanical stops are employed to limit the steering interval, which means that when mechanically stopped the steering drive is stalled and an overload safety means is activated.
In another prior art power steering system the position of the wheel or wheels is controlled and monitored by an angle sensor associated with the servo motor of the system. This is combined with a calibration means for identifying a reference point when starting up the system and for enabling a repeated checking of the angle sensor calibration during operation of the vehicle. This known device comprises a disc shaped element which rotates conjointly with the steerable wheel or wheels of the vehicle and which is provided with a semi-circular slot, and two contact-free digital sensors arranged to detect the ends of the slot. One of the sensors indicates repeatedly a reference point corresponding to the straight-ahead position of the wheel or wheels, whereas the other sensor indicates the two extreme end positions of the intended steering interval. A drawback of this known device is the need of two sensors to obtain a proper information of the actual wheel positions, which means two sources of possible malfunction.
The above mentioned problems and disadvantages are overcome by the invention.